In the continuous casting process molten metal is poured from a ladle into a reservoir (tundish) at the top of the casting device. It then passes through a submerged or a free tapping nozzle at a controlled rate into a water-cooled mould where the outer shell of the metal becomes solidified, producing a metal strand with a solid outer shell and a liquid core. Once the shell has a sufficient thickness the partially solidified strand is drawn down into a series of rolls and water sprays to further extract heat from the strand surface, which ensures that the strand is both rolled into shape and fully solidified at the same time. As the strand is withdrawn (at the casting speed) liquid metal pours into the mould to replenish the withdrawn metal at an equal rate.
Once the strand is fully solidified it is straightened and cut to the required length for example into slabs (long, thick, flat pieces of metal with a rectangular cross section), blooms (a long piece of metal with a square cross section) or billets (similar to blooms but with a smaller cross section) depending on the design of the continuous casting device.
Slag is used to remove impurities from the metal, to protect the metal from atmospheric oxidation and to thermally insulate the metal. The slag also provides lubrication between the mould walls and the solidified shell. The mould is usually also oscillated to minimize friction and sticking of the solidifying shell to the mould walls and to avoid shell tearing.
Inside the mould the flow circulates within the sides of the walls of solidifying metal. When a submerged entry nozzle is used a primary flow is generated that flows downwards in the casting direction as well as a secondary flow that flows upwards along the walls of the mould towards the meniscus i.e. the surface layer of the liquid metal in the mould.
The molten metal entering the mould carries impurities such as oxides of aluminum, calcium and iron so a noble gas such as argon is usually injected into the nozzle to prevent it from clogging with such deposits. These impurities can either float to the top of the mould in the secondary flow where they become entrained harmlessly onto the slag layer at the meniscus, often after circulating within the mould, or they can be carried down into the lower parts of the mould in the primary flow and become trapped in the solidifying front leading to defects in the cast metal products.
The metal flow into the mould must be controlled to enhance the flotation of the impurities and to prevent turbulence from drawing impurities back down into the mould where they can be incorporated into the cast products. This is usually done by applying one or more magnetic fields to act on the liquid metal entering the mould as well as on the liquid metal inside the mould. An electromagnetic brake (EMBR) can be used to slow down the liquid metal entering the mould to prevent the molten metal from penetrating deep into the cast strand. This prevents non-metallic particles and/or gas being drawn into and entrapped in the solidified strand and also prevents hot metal from disturbing the thermal and mass transport conditions during solidification causing the solidified skin to melt.
Electromagnetic stirring means can also be used to ensure a sufficient heat transport to the meniscus to avoid freezing as well as to control the flow velocity at the meniscus so that the removal of gas bubbles and inclusions from the melt is not put at risk.
If the metal flow velocity at the surface of the meniscus is too great it may shear off some of the slag layer and thereby form another source of harmful inclusions if they become entrapped in the cast products. However if the surface flow is too slow the mould powder at the meniscus may cool to a too low temperature and solidify thus decreasing its effectiveness.
Periodic velocity variations of the metal flow in the mould occur due to the oscillation of the mould, changes in the flow rate of liquid metal leaving the nozzle and variations of the casting speed. These velocity variations give rise to pressure and height variations at the meniscus which can result in slag being drawn into the lower part of the mould, an uneven slag thickness and a risk of crack formation. The velocity of the flow at the meniscus is therefore critical for both removal of impurities and trapping of slag powder and thereby related to the quality of the cast products. EP 0707909 discloses that the flow velocity at the meniscus, vm, should be maintained within the range of 0.2-0.4 ms−1 for a continuous casting process. However vm is difficult to measure directly.
U.S. Pat. No. 6,494,249 discloses a method for continuous or semi-continuous casting of a metal wherein the secondary flow velocity is monitored so that upon detection of a change in the secondary flow, information on the detected change is fed to a control unit where the change is evaluated and the magnetic flux density of the electromagnetic brake of a casting device is regulated to maintain or adjust the flow velocity. This method is based on the assumption that the flow at the meniscus, vm, is a function of the upwardly directed secondary flow.
U.S. Pat. No. 6,494,249 describes that the upwardly directed secondary flow velocity at one of the mould's sides can be monitored by monitoring the height, location and/or shape of a standing wave, that is generated on the meniscus by the upwardly directed secondary flow at one of the mould's sides. Upon detection of a change, the change is evaluated and the magnetic flux density is regulated based on this evaluation.
A disadvantage with this method is that the standing wave has to be monitored over a period of time in order to detect a change before information indicating that a change has occurred can be fed to the control unit. Oscillation of the mould during the monitoring period can affect the height, shape and location of the standing wave and thus adversely affect the accuracy of the monitoring.
Furthermore, U.S. Pat. No. 6,494,249 describes the use of electromagnetic induction sensors to monitor the standing wave. Electromagnetic induction sensors operate by detecting changes in sensor coil impedance (active or reactive), which varies as a result of changing distance between the sensor coil and the surface of a conductive material. A coil driven by a time-varying current generates a magnetic field around the sensor coil. When a ferromagnetic material is introduced into this field the coil's inductive reactance is usually increased due to the high permeability of the ferromagnetic material. A problem with using sensors that are based on electromagnetic induction is that they can experience interference from electromagnetic means such as the EMBR or stirring apparatus that are usually used in casting devices, which affects the accuracy of such sensors.